Process for separating off nitrogen

ABSTRACT

A process for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification a nitrogen-rich fraction and a methane-rich fraction, is described. 
     According to the invention, during an interruption of the supply of the feed fraction, at least temporarily, the nitrogen-rich fraction ( 4″ ) and the methane-rich fraction ( 5″ ) are compressed and jointly supplied to the process as feed fraction, wherein the compression of the nitrogen-rich fraction ( 4″ ) and the methane-rich fraction ( 5″ ) can be performed separately and/or jointly.

The invention relates to a process for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification a nitrogen-rich fraction and a methane-rich fraction.

A process of the type in question for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons is explained below with reference to the process shown in FIG. 1.

The feed fraction containing essentially nitrogen and hydrocarbons and which originates, for example, from an upstream LNG plant, is introduced via line 1. It preferably has a pressure of greater than 25 bar and was subjected, optionally to a pretreatment such as sulphur removal, carbon dioxide removal, drying etc. In the heat exchanger E1 it is cooled and partially condensed against process streams which will be considered in more detail hereinafter. Via line 1′ the partially condensed feed fraction is subsequently fed to a preseparation column T1.

This preseparation column T1 forms, together with the low-pressure column T2, a double column T1/T2. The separation columns T1 and T2 are thermally coupled via the condenser/reboiler E3.

From the bottom phase of the preseparation column T1, via line 2 a hydrocarbon-rich liquid fraction is taken off, subcooled in the heat exchanger E2 against process streams, which will be considered in more detail hereinafter, and subsequently fed via line 2′ and expansion valve a to the low-pressure column T2 in the upper region.

Via line 3, a liquid nitrogen-rich fraction is taken off from the upper region of the preseparation column T1. A substream of this fraction is added via line 3′ as reflux to the preseparation column T1. The nitrogen-rich fraction taken off via line 3 is subcooled in the heat exchanger E2 and fed via the line 3″ and expansion valve b to the low-pressure column T2 above the feed-in point of the above-described methane-rich fraction.

Via line 4, a nitrogen-rich gas fraction is taken off at the top of the low-pressure column T2. The methane content thereof is typically less than 1% by volume. In the heat exchangers E2 and E1 the nitrogen-rich fraction is subsequently warmed and optionally superheated before it is taken off via line 4″ and released into the atmosphere or optionally fed to another use.

Via line 5, a methane-rich liquid fraction is taken off from the bottom phase of the low-pressure column T2, which liquid fraction, in addition to methane, contains the higher hydrocarbons contained in the feed fraction. The nitrogen content thereof is typically less than 5% by volume. The methane-rich fraction is pumped by means of the pump P to a pressure as high as possible—this is customarily between 5 and 15 bar. In the heat exchanger E2, the methane-rich liquid fraction is warmed and optionally partially vaporized. Via line 5′ it is subsequently fed to the heat exchanger E1 and in this completely vaporized and superheated against the feed fraction which is to be cooled.

By means of the compressor V, the methane-rich fraction is subsequently compressed to the desired delivery pressure which generally corresponds to the pressure of the feed gas fraction in the line 1, and taken off from the process via line 5″.

Processes of the type in question for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons are implemented in what are termed nitrogen rejection units (NRUs). Separating off nitrogen from nitrogen/hydrocarbon mixtures is always carried out when an elevated nitrogen content impedes the intended use of the nitrogen/hydrocarbon mixture. For instance, a nitrogen content of greater than 5 mol %, for example, exceeds typical specifications of natural gas pipelines in which the nitrogen/hydrocarbon mixture is transported. Gas turbines can also only be operated up to a certain nitrogen content in the combustion gas.

Such NRUs are generally similar to an air fractionator having a double column, such as described, for example, with reference to FIG. 1, constructed as a central process unit and generally arranged in what is termed a cold box. In the case of large plant capacities, generally a plurality of cold boxes arranged in parallel are used.

Depending on the field of use, the availability of an NRU can be of great importance. An obstacle to high availability is the long time period which is required in order to restart the process after a relatively long-lasting outage of the feed fraction (NRU feed gas) containing essentially nitrogen and hydrocarbons. Outages of the NRU feed gas can occur, depending on the upstream processes or plants, several times per year, for example due to the outage of an upstream NRU-feed gas compressor or an upstream LNG/NGL plant.

In this context, a distinction must be made between restarting from the warm state (warm start-up) and the cold state (cold restart). The warm start-up is comparatively time-consuming, since all of the equipment must again be cooled to cryogenic temperatures and the liquid levels in the process must be built up. A cold restart after comparatively short outages of the NRU feed gas—these are taken to mean outage times between 1 and 24 h, wherein these depend on the ambient conditions, the size of the cold box, the type of construction and mass of the heat exchangers and also the strategy for cold restart (with/without liquids from the process)—from the cold state, can, in contrast, be carried out relatively quickly.

During a stoppage of the NRU, owing to unavoidable insulation losses, warming of the separation column(s) and also the heat exchangers, lines, etc., can occur. After a certain warming time which is determined by the plant size and the ambient conditions, a cold restart is no longer possible. The reason for this is the inevitably occurring impermissible mechanical stresses which occur when the (partially) warmed heat exchangers are charged with cold liquids or gases from the process. In such a case the NRU must therefore be warmed to ambient temperature before a warm start-up can be carried out.

In the case of longer outages of the NRU feed gas which can be caused by plant faults or maintenance work, the NRU must therefore be completely warmed before a time-consuming warm start-up can be carried out. This procedure can last, in some circumstances, longer than one week. This long warm start-up time is lost as production time and can therefore lead to considerable financial losses. This is the case, in particular, when the NRU is integrated into other plants, the production of which is dependent on the functionality of the NRU; those which may be mentioned by way of example are LNG plants having a combustion gas treatment for gas turbines by the NRU.

The object of the present invention is to specify a process of the type in question for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons which avoids the above-described disadvantages.

For achieving this object, a process of the type in question is proposed for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, which is characterized in that, during an interruption of the supply of the feed fraction, at least temporarily, the nitrogen-rich fraction and the methane-rich fraction are compressed and jointly supplied to the process as feed fraction, wherein the compression of the nitrogen-rich fraction and the methane-rich fraction can be performed separately and/or jointly.

In principle, 3 alternative procedures can be achieved thereby:

-   -   mixing the two fractions and subsequent joint compression     -   separate compression of both fractions and subsequent mixing of         the two fractions     -   separate compression of both fractions, mixing and subsequent         joint compression of both fractions

Further advantageous embodiments of the process according to the invention for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons which are subjects of the dependent claims are characterized in that

-   -   if at least one compressor (methane compressor) which         compressses the methane-rich fraction in standard operation to         the desired delivery pressure is provided, the nitrogen-rich         fraction and the methane-rich fraction are compressed by means         of the methane compressor,     -   if the feed fraction, upstream of the supply into the process,         is compressed by means of at least one compressor (feed         compressor), the nitrogen-rich fraction and the methane-rich         fraction are compressed by means of the feed compressor, and     -   the nitrogen-rich fraction and/or the methane-rich fraction are         compressed using a compressor which is without function in         standard operation.

According to the invention, during an interruption of the supply of the feed fraction, the nitrogen-rich fraction and the methane-rich fraction are then no longer delivered by the NRU, but are compressed, mixed and supplied to the NRU as substitute feed fraction. The NRU and the process of the type in question can therefore be operated virtually completely in a closed circuit. In principle, certain losses of feed gas must be expected owing to leaks and also pressure-limiting flare controllers. In order to compensate for these losses, a controlled make up stream which is mixed from nitrogen and methane is provided.

The process according to the invention for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons and also other advantageous embodiments of the same which are subjects of the dependent claims may be described in more detail hereinafter with reference to the exemplary embodiments shown in FIG. 2.

The exemplary embodiment shown in FIG. 2 differs from that shown in FIG. 1 only by the two lines 6 and 7 and also the valves c to f.

For implementing the above-described circuit operation, the delivery of the nitrogen-rich fraction 4″ is interrupted by closing the valve c and the nitrogen-rich fraction, instead, with valve d open is added via the line 6 to the methane-rich fraction. The mixed fractions are compressed in the compressor V to the desired and/or required plant pressure and subsequently, with valve f open—the methane delivery valve e is closed—supplied via the lines 7 and 1 again to the fractionation process.

Should a compressor V as shown in FIGS. 1 and 2 not be provided, a compressor suitable for the circuit operation would be supplied which then would be used exclusively for compressing the two fractions during an interruption of the supply of the feed fraction.

If an NRU is integrated in an LNG plant, generally compression of the feed fraction to be supplied to the NRU is provided. The compressor provided therefor can then be used in the procedure according to the invention for compressing the nitrogen-rich fraction and the methane-rich fraction which are combined upstream of the compressor. This advantageous embodiment of the process according to the invention is especially advantageous when a methane compressor V as shown in FIGS. 1 and 2 is not provided.

The above-described embodiment, in addition, has the advantage that the feed gas compressor—in LNG plants this is what is termed the end flash gas compressor—draws in at ambient pressure. This means that the operating pressure of the low-pressure column need not be increased, which, compared with compression of the circulated fractions in the circuit by means of the methane compressor, leads to a smaller effect on the operation of the NRU. Thus, for example, the nitrogen-rich fraction and also the methane-rich fraction taken off from the process continue to meet the product requirements of standard operation, which is not possible when the pressure of the low-pressure column is increased. This fact shortens the transition time between the “interruption operation” and standard operation.

By means of the procedure according to the invention, even after relatively long interruptions in the supply of the NRU feed gas, a rapid resumption of standard operation can now be achieved, since the operation of the separation process in the closed circuit is maintained and warming the process or the NRU are thereby avoided.

The increased expenditure in terms of apparatus and processing required for the process according to the invention are comparatively small, and so the advantages achieved by the process according to the invention certainly justify this increased expenditure. 

1. Process for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification a nitrogen-rich fraction and a methane-rich fraction, characterized in that, during an interruption of the supply of the feed fraction, at least temporarily, the nitrogen-rich fraction (4″) and the methane-rich fraction (5″) are compressed and jointly supplied to the process as feed fraction, wherein the compression of the nitrogen-rich fraction (4″) and the methane-rich fraction (5″) can be performed separately and/or jointly.
 2. Process according to claim 1, wherein at least one compressor (methane compressor) which compresses the methane-rich fraction in standard operation to the desired delivery pressure is provided, characterized in that the nitrogen-rich fraction (4″) and the methane-rich fraction (5″) are compressed by means of the methane compressor (V).
 3. Process according to claim 1, wherein the feed fraction, upstream of the supply into the process, is compressed by means of at least one compressor (feed compressor), characterized in that the nitrogen-rich fraction (4″) and the methane-rich fraction (5″) are compressed by means of the feed compressor.
 4. Process according to claim 1, characterized in that the nitrogen-rich fraction (4″) and/or the methane-rich fraction (5″) are compressed using a compressor which is without function in standard operation. 